The present invention relates to machines for cutting stacks of paper sheets or the like, and more particularly to improvements in guillotine type cutting or severing machines wherein a hold-down device in the form of a beam or the like is used to exert pressure upon the stack in the course of the severing operation so as to prevent uncontrolled stray movements of sheets and/or of the entire stack. The invention also relates to improvements in a method of cutting stacks of overlapping sheets in a machine of the above outlined character.
The quality of cuts which are made in a guillotine type severing machine depends to a considerable extent on the magnitude of the force with which the stack is held in the course of the severing operation. If the bias of the hold-down device is excessive, the device is likely to deface and/or otherwise damage the topmost sheet or sheets of the stack. On the other hand, the knife is likely to move one or more sheets or the entire stack relative to the support if the bias of the hold-down device is insufficient. The magnitude of the force which is to be applied by the hold-down device also depends on the characteristics of the material of the sheets, i.e., relatively soft material must be held down with a greater force than a relatively hard stock. Still further, the magnitude of the force which is applied by the hold-down device depends on the size (format) of the stack to be cut, and more particularly on the length of the proposed cut.
Conventional guillotine type cutting machines are provided with adjusting means for selecting the bias which the hold-down device applies to the material to be severed in the course of the cutting operation. The adjusting means is set by hand which is not satisfactory in many cases, e.g., because the quality of the severing operation is overly dependent upon the experience and/or conscientiousness of the operator. Moreover, this necessitates the presence of an attendant whenever it becomes necessary to shift from the cutting of one type of stock to the cutting of a different type of stock. Typical examples of materials which are to be cut in a guillotine type machine are stacks of sheets each of which consists of a large number of labels. A large stack is first subdivided into two or more smaller stacks, and each smaller stack is thereupon cut again so that it ultimately yields a number of stacks each containing overlapping single labels. If the subdivision of a large stack into stacks of individual labels is to take place in a series of successive operations, the bias of the hold-down device must be adjusted upon completion of each cut or, at the very least, at relatively frequent intervals. This is a cumbersome and time-consuming operation.
German Auslegeschrift No. 1 093 774 discloses a cutting machine wherein the force with which the hold-down device bears upon a stack is assumed to be dependent upon the length of the cut, and such force can be selected in response to signals which are generated by means for monitoring the format of the stack. This mode of operation allows for a certain degree of automation but is not entirely satisfactory because the sensors merely monitor the length of the cut to be made but cannot adequately take into consideration other parameters which affect the quality of the cutting operation.